1. Field of the Invention
The present invention relates to a passive device and fabrication method thereof, and in particularly to a fabrication method for a solid electrolytic capacitor and coupling agent utilized in the same.
2. Description of the Related Art
A major object of electrolytic capacitors' development is to improve the conductivity of an electrolyte in order to reduce the equivalent series resistance (ESR) thereof, thereby achieving the properties of low resistance at high frequency and superior reliability. Conducting polymers have higher conductivity than liquid electrolyte, solid organic semiconductor complex salts such as TCNQ, or manganese oxide used in conventional electrolytic capacitors, and can further serve as insulators at high temperature. Therefore, conducting polymers are currently the most popular materials used for solid electrolytes in electrolytic capacitors.
FIG. 1 is a zoom-in cross-section of a portion of the micro structure of a solid electrolytic capacitor. A valve metal 100 is a meso-porous material acting as a positive electrode. A dielectric layer 110 is disposed on the valve metal 100. A conducting polymer layer 120, acting as a negative electrode, is disposed on the dielectric layer 110. The conducting polymer layer 120 bonds to the dielectric layer 110 with only van der Waal's force, which is too weak to prevent the formation of a void 121 between the dielectric layer 110 and conducting polymer layer 120, thus the electrical performance of the capacitor suffers, resulting in loss of capacitance, increases in ESR and dissipation factor (DF), and further negatively affecting the reliability of the capacitor.
Sato et al., in JP9246106, disclose a before-treatment procedure using a silane coupling agent, such as gamma-glycidoxypropyltrimethoxysilane or octadecyl-triethoxysilane, on a forming aluminum foil, followed by forming a conducting polymer layer thereon. The coupling agent used therein, however, comprises functional groups which negatively affect the conductivity of the conducting polymer, resulting in the increase of ESR and DF.
Sakata et al., in U.S. Pat. No. 5,729,428, disclose a before-treatment procedure using an organic compound, such as organic acid, phenol, silane coupling agent, aluminum coupling agent, and titanium coupling agent, followed by forming a conducting polymer layer thereon, in order to improve leakage at high temperature. Hahn et al., in U.S. Pat. No. 6,072,694, disclose an electrolytic capacitor whose adhesion of a conducting polymer film to an oxidized porous pellet anode is improved by the incorporation of a silane coupling agent in the polymer impregnating solution, in order to improve leakage and dissipation factor thereof. The coupling agents used by these two arts, however, cannot bond to both conducting polymers and dielectric materials, thus limiting the effect of assisting the connection of conducting polymers and dielectric materials.